diff --git a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
index ad8435842b..144a560f7f 100644
--- a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
@@ -458,8 +458,9 @@ internal static class ChatResponseUpdateAGUIExtensions
             // This ensures all AGUI events have a valid messageId regardless of agent type.
             if (string.IsNullOrWhiteSpace(chatResponse.MessageId))
             {
-                streamingMessageId ??= Guid.NewGuid().ToString("N");
-                chatResponse.MessageId = streamingMessageId;
+                chatResponse.MessageId = ContainsToolResult(chatResponse)
+                    ? Guid.NewGuid().ToString("N")
+                    : (streamingMessageId ??= Guid.NewGuid().ToString("N"));
             }
 
             if (chatResponse is { Contents.Count: > 0 } &&
@@ -725,4 +726,17 @@ internal static class ChatResponseUpdateAGUIExtensions
             _ => JsonSerializer.Serialize(functionResultContent.Result, options.GetTypeInfo(functionResultContent.Result.GetType())),
         };
     }
+
+    private static bool ContainsToolResult(ChatResponseUpdate chatResponse)
+    {
+        foreach (AIContent content in chatResponse.Contents)
+        {
+            if (content is FunctionResultContent)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs
index 5c55408ff8..502e23d81c 100644
--- a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs
@@ -149,6 +149,93 @@ public sealed class AGUIStreamingMessageIdTests
             "ParentMessageId should have a generated fallback for empty provider MessageId");
     }
 
+    /// <summary>
+    /// Tool results are separate tool-role messages, so their fallback IDs must not
+    /// collide with the assistant message that requested the tool call.
+    /// </summary>
+    [Fact]
+    public async Task ToolResults_NullMessageId_GeneratesDistinctMessageIdAsync()
+    {
+        FunctionCallContent functionCall = new("call_abc123", "GetWeather")
+        {
+            Arguments = new Dictionary<string, object?> { ["location"] = "San Francisco" }
+        };
+
+        List<ChatResponseUpdate> providerUpdates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Checking the weather"),
+            new ChatResponseUpdate
+            {
+                Role = ChatRole.Assistant,
+                Contents = [functionCall]
+            },
+            new ChatResponseUpdate(ChatRole.Tool, [new FunctionResultContent("call_abc123", "72F and sunny")])
+        ];
+
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in providerUpdates.ToAsyncEnumerableAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        TextMessageStartEvent textStart = Assert.Single(aguiEvents.OfType<TextMessageStartEvent>());
+        ToolCallStartEvent toolCallStart = Assert.Single(aguiEvents.OfType<ToolCallStartEvent>());
+        ToolCallResultEvent toolCallResult = Assert.Single(aguiEvents.OfType<ToolCallResultEvent>());
+
+        Assert.Equal(textStart.MessageId, toolCallStart.ParentMessageId);
+        Assert.Equal("call_abc123", toolCallResult.ToolCallId);
+        Assert.False(string.IsNullOrEmpty(toolCallResult.MessageId));
+        Assert.NotEqual(textStart.MessageId, toolCallResult.MessageId);
+    }
+
+    [Fact]
+    public async Task ToolResults_WithTextContent_GeneratesDistinctMessageIdAsync()
+    {
+        FunctionCallContent functionCall = new("call_abc123", "GetWeather")
+        {
+            Arguments = new Dictionary<string, object?> { ["location"] = "San Francisco" }
+        };
+
+        List<ChatResponseUpdate> providerUpdates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Checking the weather"),
+            new ChatResponseUpdate
+            {
+                Role = ChatRole.Assistant,
+                Contents = [functionCall]
+            },
+            new ChatResponseUpdate
+            {
+                Role = ChatRole.Tool,
+                Contents =
+                [
+                    new TextContent("Tool says: "),
+                    new FunctionResultContent("call_abc123", "72F and sunny")
+                ]
+            }
+        ];
+
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in providerUpdates.ToAsyncEnumerableAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        TextMessageStartEvent[] textStarts = aguiEvents.OfType<TextMessageStartEvent>().ToArray();
+        TextMessageContentEvent toolText = Assert.Single(
+            aguiEvents.OfType<TextMessageContentEvent>(),
+            content => content.Delta == "Tool says: ");
+        ToolCallStartEvent toolCallStart = Assert.Single(aguiEvents.OfType<ToolCallStartEvent>());
+        ToolCallResultEvent toolCallResult = Assert.Single(aguiEvents.OfType<ToolCallResultEvent>());
+
+        Assert.Equal(textStarts[0].MessageId, toolCallStart.ParentMessageId);
+        Assert.NotEqual(textStarts[0].MessageId, toolCallResult.MessageId);
+        Assert.Equal(toolCallResult.MessageId, toolText.MessageId);
+        Assert.Equal(textStarts[^1].MessageId, toolCallResult.MessageId);
+    }
+
     /// <summary>
     /// When a provider properly sets MessageId (e.g., OpenAI), the AGUI pipeline
     /// produces valid events with correct messageId values.